1. Field of the Invention
The invention relates to a solid-state imaging device that is used in a camera and the like.
2. Background Art
In recent years, solid-state imaging devices have been actively developed and their applications have been extended to various fields such as an in-vehicle camera and a surveillance camera in addition to a video camera and a digital still camera. An in-vehicle camera and a surveillance camera are sometimes used in a very severe environment as compared to a video camera, a digital still camera, and the like. For example, a surveillance camera that is installed outdoor, an in-vehicle camera that is mounted near a driving portion of a vehicle, and the like are expected to be used in an environment in which an ambient temperature changes significantly.
In commonly used CCD (Charge Coupled Device) type and MOS (Metal Oxide Semiconductor) type solid-state imaging devices, a dark current increases with an increase in temperature of the solid-state imaging device, which may cause significant degradation in image quality. Moreover, an increase in temperature of the solid-state imaging device itself may possibly reduce the service life of the solid-state imaging device. For example, an in-vehicle camera is used to control steering of a vehicle by detecting a white line on the road that continuously extends from far away to near the vehicle and to control braking of a vehicle by recognizing an environment ahead of the vehicle such as a road structure and a vehicle in front. Since degradation in image quality due to an increase in temperature of a solid-state imaging device may cause malfunction of the above control operations, a high quality level is required for a solid-state imaging device.
In view of the above problems, Japanese Laid-Open Patent Publication No. 2-44872, for example, proposes a technology of detecting a temperature of a solid-state imaging device by utilizing temperature dependency of a differential output (a dark current) between an output of a photoelectric conversion portion of an optical black portion provided in the solid-state imaging device and an empty feed output that does not include an output signal of the photoelectric conversion portion. In this technology, the temperature of the solid-state imaging device is kept constant by operating a cooling device based on the detected temperature information, whereby degradation in image quality is prevented.